Fuel assemblies for nuclear reactors and in particular the fuel assemblies for water nuclear reactors consist of a bundle of parallel fuel rods held by a structure consisting of spacer grids, two end plates, and guide tubes which connect the spacer grids and the end plates.
The fuel rods, which are cylindrical and of great length, are engaged in the spacer grids which hold them in position in the bundle.
The spacer grids generally provide both the transverse positioning and the axial positioning of the rods which are shorter than the guide tubes which are substituted for certain rods of the assembly. The ends of the rods are therefore free and are located at a certain distance from the end plates.
In order to provide both the axial positioning and the transversal positioning of the rods by virtue of the spacer grids, it is necessary to use springs which exert a high transversal force on the rods.